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Carbon nanotube

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For the first time, scientists have built a transistor out of carbon nanotubes that can run almost twice as fast as its silicon counterparts.

This is big, because for decades, scientists have been trying to figure out how to build the next generation of computers using carbon nanotube components, because their unique properties could form the basis of faster devices that consume way less power.

“Making carbon nanotube transistors that are better than silicon transistors is a big milestone,” said one of the team, Michael Arnold, from the University of Wisconsin-Madison. “This achievement has been a dream of nanotechnology for the last 20 years.”

First developed back in 1991, carbon nanotubes are basically minuscule carbon straws that measure just 1 atom thick.

Imagine a tiny, cylindrical tube that’s approximately 50,000 times smaller than the width of a human hair, and made from carbon atoms arranged in hexagonal arrays. That’s what a carbon nanotube wire would look like if you could see it at an atomic level.

Because of their size, carbon nanotubes can be packed by the millions onto wafers that can act just like a silicon transistor – the electrical switches that together form a computer’s central processing unit (CPU).

Despite being incredibly tiny, carbon nanotubes have some unique properties that make them an engineer’s dream.

And here’s the best part: just like that other 1-atom-thick wonder-material, graphene, carbon nanotubes are one of the most conductive materials ever discovered.

With ultra-strong bonds holding the carbon atoms together in a hexagonal pattern, carbon nanotubes are able to produce a phenomenon known as electron delocalisation, which allows an electrical charge to move freely through it.

The arrangement of the carbon atoms also allows heat to move steadily through the tube, which gives it around 15 times the thermal conductivity and 1,000 times the current capacity of copper, while maintaining a density that’s just half that of aluminium.

Because of all these amazing properties, these semiconducting powerhouses could be our answer to the rapidly declining potential of silicon-based computers.

Right now, all of our computers are running on silicon processors and memory chips, but we’ve about hit the limit for how fast these can go. If scientists can figure out how to replace silicon-based parts with carbon nanotube parts, in theory, we could bump speeds up by five times instantly.

But there’s a major problem with mass-producing carbon nanotubes – they’re incredibly difficult to isolate from all the small metallic impurities that creep in during the manufacturing process, and these bits and pieces can interrupt their semiconducting properties.

But Arnold and his team have finally figured out how to get rid of almost all of these impurities. “We’ve identified specific conditions in which you can get rid of nearly all metallic nanotubes, where we have less than 0.01 percent metallic nanotubes,” he says.

As Daniel Oberhaus explains for Motherboard, the technique works by controlling the self-assembling properties of carbon nanotubes in a polymer solution, which not only allows the researchers to clean out impurities, but also to manipulate the proper spacing of nanotubes on a wafer.

“The end result are nanotubes with less than 0.01 percent metallic impurities, integrated on a transistor that was able to achieve a current that was 1.9 times higher than the most state-of-the-art silicon transistors in use today,” he says.

Simulations have suggested that in their purest form, carbon nanotube transistors should be able to able to perform five times faster or use five times less energy than silicon transistors, because their ultra-small dimensions allow them to very quickly switch a current signal as it travels across it.

This means longer-lasting phone batteries, or much faster wireless communications or processing speeds, but scientists have to actually build a working computer filled with carbon nanotube transistors before we can know for sure.

Arnold’s team has already managed to scale their wafers up to 2.5 by 2.5 cm transistors (1 inch by 1 inch), so they’re now figuring out how to make the process efficient enough for commercial production.

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A new version of “spaser” technology being investigated could mean that mobile phones become so small, efficient, and flexible they could be printed on clothing.
A team of researchers from Monash University’s Department of Electrical and Computer Systems Engineering (ECSE) has modelled the world’s first spaser (surface plasmon amplification by stimulated emission of radiation) to be made completely of carbon.
A spaser is effectively a nanoscale laser or nanolaser. It emits a beam of light through the vibration of free electrons, rather than the space-consuming electromagnetic wave emission process of a traditional laser.
PhD student and lead researcher Chanaka Rupasinghe said the modelled spaser design using carbon would offer many advantages.
“Other spasers designed to date are made of gold or silver nanoparticles and semiconductor quantum dots while our device would be composed of a graphene resonator and a carbon nanotube gain element,” Chanaka said.
“The use of carbon means our spaser would be more robust and flexible, would operate at high temperatures, and be eco-friendly.
“Because of these properties, there is the possibility that in the future an extremely thin mobile phone could be printed on clothing.”
Spaser-based devices can be used as an alternative to current transistor-based devices such as microprocessors, memory, and displays to overcome current miniaturising and bandwidth limitations.
The researchers chose to develop the spaser using graphene and carbon nanotubes. They are more than a hundred times stronger than steel and can conduct heat and electricity much better than copper. They can also withstand high temperatures.
Their research showed for the first time that graphene and carbon nanotubes can interact and transfer energy to each other through light. These optical interactions are very fast and energy-efficient, and so are suitable for applications such as computer chips.
“Graphene and carbon nanotubes can be used in applications where you need strong, lightweight, conducting, and thermally stable materials due to their outstanding mechanical, electrical and optical properties. They have been tested as nanoscale antennas, electric conductors and waveguides,” Chanaka said.

Chanaka said a spaser generated high-intensity electric fields concentrated into a nanoscale space. These are much stronger than those generated by illuminating metal nanoparticles by a laser in applications such as cancer therapy.
“Scientists have already found ways to guide nanoparticles close to cancer cells. We can move graphene and carbon nanotubes following those techniques and use the high concentrate fields generated through the spasing phenomena to destroy individual cancer cells without harming the healthy cells in the body,” Chanaka said

Scientists at Columbia University conducted a study which revealed that graphene retains most of its mechanical properties even when it has been stitched together from small fragments. This discovery may have been the first step toward large scale manufacture of carbon nanotubes, which could be essential in the manufacturing of the first space elevator, light – strong materials, and flexible electronics.

At the present moment, a practical breakthrough in the construction of a lunar elevator has not been realized. However, many scientists have performed experiments which show it will be possible through use of graphene. In these experiments, they have measured the strength of the microscopic carbon nanotube and proved it can indeed support the construction of such elevators.

The space elevator ribbon is constructed out of carbon nanotubes, which are at least 100 times stronger than steel but have flexibility equal to that of plastic. Scientists will only be able to make the ribbon to be used in the space elevator if they manage to make fibers out of carbon nanotubes. In the recent experiments, the materials that were involved were neither strong nor flexible enough to form such a ribbon.

Grapheneribbons have a very high tensile strength and very high elastic modulus, theoretically they are said to make the process of building a space elevator easy. There are two major ways that a lunar elevator ribbon can be built: in the first case a long carbon tube ideally several meters long will be braided into a rope like structure, and in the second case a shorter nanotube will be placed in a selected polymer matrix.

So far graphene is the ideal material for construction of the ribbon, the carbon-carbon bond in graphene is at least 0.142 nm. Scientists have proved that two sheets of graphene are held together by much stronger van de Waals forces than bulk Graphene.

But while single-nanotube transistors have been around for 15 years, no-one had ever put the jigsaw pieces together to make a useful computing device.

So how did the Stanford team succeed where others failed? By overcoming two common bugbears which have bedevilled carbon computing.

First, CNTs do not grow in neat, parallel lines. “When you try and line them up on a wafer, you get a bowl of noodles,” says Mitra.

The Stanford team built chips with CNTs which are 99.5% aligned – and designed a clever algorithm to bypass the remaining 0.5% which are askew.

They also eliminated a second type of imperfection – “metallic” CNTs – a small fraction of which always conduct electricity, instead of acting like semiconductors that can be switched off.

To expunge these rogue elements, the team switched off all the “good” CNTs, then pumped the remaining “bad” ones full of electricity – until they vaporised. The result is a functioning circuit.

The Stanford team call their two-pronged technique “imperfection-immune design”. Its greatest trick? You don’t even have to know where the imperfections lie – you just “zap” the whole thing.

“These are initial necessary steps in taking carbon nanotubes from the chemistry lab to a real environment,” said Supratik Guha, director of physical sciences for IBM’s Thomas J Watson Research Center.

But hang on – what if, say, Intel, or another chip company, called up and said “I want a billion of these”. Could Cedric be scaled up and factory-produced?

A team of Stanford engineers has built a basic computer using carbon nanotubes, a semiconductor material that has the potential to launch a new generation of electronic devices that run faster, while using less energy, than those made from silicon chips.

This unprecedented feat culminates years of efforts by scientists around the world to harness this promising material.

The achievement is reported today in an article on the cover of Nature magazine written by Max Shulaker and other doctoral students in electrical engineering. The research was led by Stanford professors Subhasish Mitra and H.S. Philip Wong.

“People have been talking about a new era of carbon nanotube electronics moving beyond silicon,” said Mitra, an electrical engineer and computer scientist, and the Chambers Faculty Scholar of Engineering. “But there have been few demonstrations of complete digital systems using this exciting technology. Here is the proof.”

But until now it hasn’t been clear that CNTs could fulfill those expectations.

“There is no question that this will get the attention of researchers in the semiconductor community and entice them to explore how this technology can lead to smaller, more energy-efficient processors in the next decade,” Rabaey said.

Mihail Roco, senior advisor for Nanotechnology at the National Science Foundation, called the Stanford work “an important, scientific breakthrough.”

It was roughly 15 years ago that carbon nanotubes were first fashioned into transistors, the on-off switches at the heart of digital electronic systems.

“First, they put in place a process for fabricating CNT-based circuits,” De Micheli said. “Second, they built a simple but effective circuit that shows that computation is doable using CNTs.”

As Mitra said: “It’s not just about the CNT computer. It’s about a change in directions that shows you can build something real using nanotechnologies that move beyond silicon and its cousins.”

Why worry about a successor to silicon? Such concerns arise from the demands that designers place upon semiconductors and their fundamental workhorse unit, those on-off switches known as transistors

For decades, progress in electronics has meant shrinking the size of each transistor to pack more transistors on a chip. But as transistors become tinier they waste more power and generate more heat – all in a smaller and smaller space, as evidenced by the warmth emanating from the bottom of a laptop.

Many researchers believe that this power-wasting phenomenon could spell the end of Moore’s Law, named for Intel Corp. co-founder Gordon Moore, who predicted in 1965 that the density of transistors would double roughly every two years, leading to smaller, faster and, as it turned out, cheaper electronics.

But smaller, faster and cheaper has also meant smaller, faster and hotter.

“Energy dissipation of silicon-based systems has been a major concern,” said Anantha Chandrakasan, head of electrical engineering and computer science at MIT and a world leader in chip research. He called the Stanford work “a major benchmark” in moving CNTs toward practical use. CNTs are long chains of carbon atoms that are extremely efficient at conducting and controlling electricity. They are so thin – thousands of CNTs could fit side by side in a human hair – that it takes very little energy to switch them off, according to Wong, co-author of the paper and the Williard R. and Inez Kerr Bell Professor at Stanford.

“Think of it as stepping on a garden hose,” Wong said. “The thinner the hose, the easier it is to shut off the flow.” In theory, this combination of efficient conductivity and low-power switching make carbon nanotubes excellent candidates to serve as electronic transistors.

“CNTs could take us at least an order of magnitude in performance beyond where you can project silicon could take us,” Wong said. But inherent imperfections have stood in the way of putting this promising material to practical use.

First, CNTs do not necessarily grow in neat parallel lines, as chipmakers would like.

Over time, researchers have devised tricks to grow 99.5 percent of CNTs in straight lines. But with billions of nanotubes on a chip, even a tiny degree of misaligned tubes could cause errors, so that problem remained.

A second type of imperfection has also stymied CNT technology.

Depending on how the CNTs grow, a fraction of these carbon nanotubes can end up behaving like metallic wires that always conduct electricity, instead of acting like semiconductors that can be switched off.

Since mass production is the eventual goal, researchers had to find ways to deal with misaligned and/or metallic CNTs without having to hunt for them like needles in a haystack.

“We needed a way to design circuits without having to look for imperfections or even know where they were,” Mitra said. The Stanford paper describes a two-pronged approach that the authors call an “imperfection-immune design.”

To eliminate the wire-like or metallic nanotubes, the Stanford team switched off all the good CNTs. Then they pumped the semiconductor circuit full of electricity. All of that electricity concentrated in the metallic nanotubes, which grew so hot that they burned up and literally vaporized into tiny puffs of carbon dioxide. This sophisticated technique was able to eliminate virtually all of the metallic CNTs in the circuit at once.

Bypassing the misaligned nanotubes required even greater subtlety.

So the Stanford researchers created a powerful algorithm that maps out a circuit layout that is guaranteed to work no matter whether or where CNTs might be askew.

“This ‘imperfections-immune design’ (technique) makes this discovery truly exemplary,” said Sankar Basu, a program director at the National Science Foundation.

The Stanford team used this imperfection-immune design to assemble a basic computer with 178 transistors, a limit imposed by the fact that they used the university’s chip-making facilities rather than an industrial fabrication process.

Their CNT computer performed tasks such as counting and number sorting. It runs a basic operating system that allows it to swap between these processes. In a demonstration of its potential, the researchers also showed that the CNT computer could run MIPS, a commercial instruction set developed in the early 1980s by then Stanford engineering professor and now university President John Hennessy.

Though it could take years to mature, the Stanford approach points toward the possibility of industrial-scale production of carbon nanotube semiconductors, according to Naresh Shanbhag, a professor at the University of Illinois at Urbana-Champaign and director of SONIC, a consortium of next-generation chip design research.

“These are initial necessary steps in taking carbon nanotubes from the chemistry lab to a real environment,” said Supratik Guha, director of physical sciences for IBM’s Thomas J. Watson Research Center and a world leader in CNT research.

Specifically, the investigators from Rice University, in cooperation with the University of Houston and St. Luke’s Episcopal Hospital, are inserting bismuth compounds into single-walled carbon nanotubes to make a more effective CT contrast agent. In tests using pig bone marrow-derived mesenchymal stem cells, the researchers found that the bismuth-filled nanotubes, which they have dubbed Bi@US-tubes, produce CT images of higher attenuation than those with iodine-based contrast agents .

Bismuth has been used before as a contrast agent, but putting it in nanotube capsules allowed the researchers to get the substance inside cells in high concentrations, permitting the acquisition of CT images of the cell. The relatively high contrast is achieved with low bismuth loading (2.66% by weight) within the tubes, without compromising cell viability.

Bismuth is a heavy element and therefore is more effective at diffracting x-rays than almost any substance, according to study co-author Lon Wilson, PhD. Going forward, the nanotube surfaces can be modified to improve biocompatibility and their ability to target certain types of cells. They can also be modified for use with MRI, PET, and electron paramagnetic resonance imaging systems, he said.

The researchers are now working to double the amount of bismuth in each nanotube. They would also like to combine bismuth and gadolinium into a single nanotube to produce a bimodal contrast agent suitable for tracking in both CT and MRI, Wilson said.

First it was diamond, then graphene. These two structures have previously held the title of the world’s strongest material. Now, one of their family members has taken the crown.

In a research paper published recently onArxiv, a team from Rice University laid out the molecular schematics for Carbyne, aka linear acetylenic carbon. A supermaterial first theorized in 1967, its legitimacy has been disputed for the last 40 years. This time around the team figured out how to successfully synthesize and stabilize it at room temperature.

The paper goes on to describe the remarkable atomic chain of Carbyne – a microscopic lattice similar to that of its close cousin, diamond. Carbyne, however, has a Young’s modulus 40 times that of diamond, making it the world’s hardest material. With extensive applications in nanotechnology, it could completely change the way scientists view systems with nanomechanical bases. The polyyne family has a new heavyweight champ.

Above: Carbyne under tension. (a) DFT calculations of energy as a function of strain ɛ. The electronic density of carbyne (polyyne) (b) in equilibrium and (c) under tension shows a more pronounced bond alternation in strained carbyne. (d) Bond length alternation and (e) band gap increase as a function of strain.

The results reveal remarkable tensile stiffness, twice that of graphene and carbon nanotubes, and a specific strength greater than any other known material. Potential mechanical and electrical applications are numerous, including a broad category of possible uses in the realm of super-strong and ultra-lightweight materials.

Now the researchers are reporting a new artificial muscle–building technique that makes their carbon nanotube yarns several times faster and more powerful. These qualities could help deliver on the technology’s promise of developing compact, lightweight actuators for robots, exoskeletons and other mechanical devices, although several challenges remain.

The latest breakthrough comes from infusing the carbon nanotube yarns with paraffin wax that expands when heated, enabling the artificial muscles to lift more than 100,000 times their own weight and generate 85 times more mechanical power during contraction than mammalian skeletal muscles of comparable size, according to the researchers, whose latest work is published in the November 16 issue of Science.

The previous-generation artificial muscles were electrochemical and functioned like a supercapacitor. When a charge was injected into the carbon nanotube yarn, ions from a liquid electrolyte diffused into the yarn, causing it to expand in volume and contract in length, says Baughman, director of the University of Texas at Dallas‘s Alan G. MacDiarmid NanoTech Institute. Unfortunately, using an electrolyte limited the temperature range in which the muscle could function. At colder temperatures the electrolyte would solidify, slowing down the muscle; if too hot, the electrolyte would degrade. It also needed a container, which added weight to the artificial-muscle system.

The wax eliminates the need for an electrolyte, making the artificial muscle lighter, stronger and more responsive. When heat or a light pulse is applied to a wax-impregnated yarn about 200 microns in diameter (roughly twice that of a human hair), the wax melts and expands. In about 25 milliseconds this expansion creates pressure causing the yarn’s individual nanotube threads to twist and the yarn’s length to contract. Any weightlifter will tell you that the success of any muscle—artificial or natural—depends in part on the degree of this contraction. Depending on the force exerted, the Baughman team’s muscle strands could contract by up to 10 percent.

Muscles are also judged by the weight they can lift relative to their size. “Our muscles can lift about 200 times the weight of a similar-size natural muscle,” Baughman says, adding that the wax-infused artificial muscles can also generate 30 times the maximum power of their electrolyte-powered predecessors.

The researchers’ latest artificial muscles move the technology closer to commercialized products such as environmental sensors, aerospace materials and even textiles that take can take advantage of nanoscale actuators, University of Cincinnati mechanical engineering professor Mark Schulz, wrote in a related SciencePerspectives article. This new artificial muscle outperforms existing ones, allowing possible applications such as linear and rotary motors; it also might replace biological muscle tissue if biocompatibility can be established, he adds.

However, Schulz points out—and Baughman is quick to acknowledge—that even this new crop of artificial muscles faces many challenges before they can be a practical alternative to mini–electric motors in many of the products we buy. Despite their improvements, the latest artificial muscles are for the most part inefficient and limited in the combinations of force, motion and speed they can generate, according to Schulz.

Indeed, these new artificial muscles operate at about 1 percent efficiency, a number Baughman and his colleagues want to increase at least 10-fold. An option for improving efficiency is to use a chemical fuel rather than electricity to power the muscles. “One way to compensate for a lack of efficiency is to use fuel like methanol instead of a battery,” he says. “You could store more than 20 percent more energy in a fuel like methanol than you can in a battery.”

Another challenge is that the artificial muscles must be heated and cooled to contract and release, respectively. Short lengths of yarn can cool on their own in a matter of seconds, but longer pieces would need to be actively cooled using water or air, otherwise the muscle would not relax. “Or you’d need [to use a] material that doesn’t require thermal actuation,” Baughman says. “If you keep making the [carbon nanotube] yarn longer and longer, your cooling rate increases.”

This issue of scale poses perhaps the greatest challenge. A one-millimeter length of artificial muscle can lift about 50 grams, according to Baughman. That means lifting several tons would require a greater length of carbon nanotube yarn than is practical. “We’d like our artificial muscles to be used in exoskeletons that help workers or soldiers lift objects weighing tons,” he says. But the researchers are still working out ways to pack enough yarn to perform such tasks into the length of an exoskeletal limb.

Carbon nanotube artificial muscles are more likely to first appear in products requiring only short lengths. Baughman envisions artificial muscles used in a catheter for minimally invasive surgery, “where you want to have lots of functionality on the end of the catheter to do surgical manipulations.” Another application with flex appeal—”smart” fabrics that can automatically react to their environments, becoming more or less porous when they detect heat or harmful chemicals in the air.